Field of the Invention
The invention relates to a power transmission technique, and more particularly, to a power transmission apparatus with over-loading protection and power-saving mechanism.
Description of Related Art
In modern life, as usage of electric appliances becoming even more popular, people's demand on electric appliances also increases each day. An extension cord is often purchased in order to satisfy more of appliances in many households and working places, and some people might even choose a smart extension cord for its added values of power-saving and electrical safety. The smart extension cord usually includes a smart switch. The smart switch can be automatically turned off or controlled to switch between on/off states when excessive power consumption or over-loading occurs.
With current technology, an actuating element of the smart switch is basically composed of relays. Common relays include an electromagnetic relay (EMR) and a solid state relay (SSR). Among them, the electromagnetic relay is capable of bearing greater current and suffering less impact from temperature variation on the electromagnetic relay. Power consumed while driving the electromagnetic relay is different based on different rated current values but is mostly of a fixed value. For example, power consumed while driving the electromagnetic relay with an AC operating voltage of 250 VAC and the rated current value of 15 A is approximately 0.5 W. However, in the case of light-loading, power consumed by the electromagnetic relay is higher in comparison with the solid state relay. Also, even in a standby mode (i.e., zero load), the electromagnetic relay will continuously consume power. On the other hand, power consumed by the solid state relay is proportional to a load current. Therefore, during the standby mode (i.e., zero load, where the load current is approximately OA) or when the load current is less than 0.5 A (i.e., light-loading), the solid state relay can provide a characteristic of zero power consumption or less power consumption for effectively reducing power consumption and a faster switching speed. Therefore, in some low power consumption (power-saving) applications, the solid state relay may be used to replace the electromagnetic relay (example 1), or the solid state relay may be used together with the electromagnetic relay (example 2). However, although the solid state relay has the characteristic of low power consumption and the faster switching speed, in the application of example 2 where the solid state relay is used together with the electromagnetic relay to realize the smart switch, if appropriate protection measures are not available, once over-loading suddenly occurs when the solid state relay is turned on, instantaneous high current from city power is likely to cause damage to the solid state relay.
Accordingly, it is one of important issues to be solved by persons skilled in the art as how to prevent the solid state relay from damages due to sudden increases in load power consumption during a power-saving mode (i.e., when the solid state relay is turned on) of the extension cord when a switch apparatus of the extension cord adopts example 2 (i.e., the solid state relay is used together with the electromagnetic relay) to achieve optimal application of both high power and power-saving.